Early printed circuit boards comprised single-sided composite circuit boards designed for mounting electronic components and connecting the components through wiring circuits running on one surface of the circuit board. As the complexity of electronic circuits grew, so did the need to make more electronic connections on a circuit board. This resulted in the manufacturing of double-sided printed circuit boards capable of having circuits and electronic connections on two surfaces of the circuit boards.
Many recent electronic systems have far more complex circuits, densely populated with multiple components and wiring traces, which are severely limited by having only two surfaces to make all the electrical connections. To create more circuit connections in a smaller circuit board area, multilayer printed circuit boards were developed.
The conventional method of manufacturing a multilayer printed circuit board involves creating circuit board connections using plated through-holes or vias. Circuit networks or traces are formed in different layers that are connected to each other at a common point where a connecting pad is placed. A hole is drilled through the connecting pad and an electrically conductive layer is added to the hole wall (e.g., using a plating or other process) so that two or more circuits on different layers are electrically connected together. In addition to making the necessary connections between layers, a hole may serve an additional function of connecting to components. That is, a hole may serve to receive a terminal or lead from an electrical component, for instance.
With the introduction of surface mounted technology, component holes make up a lesser quantity of all drilled holes in highly complex circuit boards. A majority of the plated through holes, also known as via holes, are mostly used for electrical connections between circuit layers.
Drilling a mechanical via hole through a stack of multilayer printed circuit boards wastes valuable board space because of the sizes of the via holes that can be cost effectively drilled, the large capture pads required for high yield manufacturing and the lost space on layers that do not need an interconnection at all of the points drilled. As a result, vertical interconnection on a layer-to-layer basis has gained in popularity among multilayer printed circuit board designers and manufacturer.
Microvias, which have a smaller opening than traditional vias, are formed using such techniques as laser, photolithography and plasma etching and have been known and/or used by designers, manufacturers, and/or fabricators. However, there is a lot of technical know-how involved in reliably and consistently manufacturing microvias. Take for example, the process of electroless copper deposition which is a common process for seeding a microvia wall before a thicker layer of electrolytic copper is plated over the microvia wall. The circuit boards or substrates through with the microvia holes are formed are typically treated with a swelling agent, a permanganate oxidizer, a reducing agent to reduce the permanganate residue, conditioned with a conditioning agent, microetched to remove the conditioning agent, catalyzed with a Palladium-Tin (Pd-Sn) colloid, treated with hydrochloric acid to expose the Pd and finally being plated. The plating solution typically contains a reducing agent (e.g., formaldehyde or hypophosphite), copper salts and a chelating agent (e.g., EDTA, alkanol amines or tartarates) to keep the copper salt in solution. These chemical processes typically employ two to three rinses in between each process. To achieve reliability and consistency, it is necessary for each chemical process and its respective rinses to perform their relevant functions correctly within the tiny microvias, not most of the time, but in every bath every time. Hence, tight process control with well designed equipment is necessary to make reliable microvias consistently.
In addition to the reliability difficulties in plating microvias, there are other setbacks. For example, chemicals trapped within the microvias may lead to outgassing during the assembly process and additional potential reliability problems.
As the electronic industry's demand for portability, smaller form factors, more built-in functions, and more sophisticated electronic systems grows, the quest to design more circuit connections within a smaller printed circuit board area continues.
U.S. Pat. No. 5,231,757, by Chantraine et al., discloses the use of via studs for a multilayer structure formed on a uniform metal layer that is subsequently etched to form conductors for the multilayer structure. A dielectric layer then covers the entire surface inclusive of the studs. The tips of the studs are then exposed through the dielectric layer by plasma or mechanical means. It is noted the dielectric employed, even though not specified, suggest a non-reinforced material. The embodiment illustrated in the patent is based on polyamic acid as a liquid coat, which is subsequently polymerized to become polyimide.
U.S. Pat. No. 5,457,881, by Schmidt, discloses the protrusions with a distal ends that penetrate a dielectric layer. Even though not specified, the patent suggests that the dielectric layer is made from a non-reinforce material, which conceptually allows the protrusions to penetrate through the dielectric layer. The use of a non-reinforced dielectric layer is undesirable for many modern circuits which tend to use fiberglass reinforced dielectric layers.
The disadvantages of the processes disclosed U.S. Pat. Nos. 5,231,757 and 5,457,881, lies on the necessity to use an appropriate dielectric. Conventional dielectric materials for printed circuit boards, commonly known as prepreg, typically include resin with glass cloth reinforcement. It is easy for conductive studs, protrusions, or bumps to penetrate a pure resin dielectric layer. However, it is relatively difficult for these conductive studs, protrusions, or bumps to penetrate the embedded glass cloth in the prepreg.
U.S. Pat. No. 5,736,681, to Yamamoto et al., discloses a method for making interconnections through a conventional reinforced prepreg layer. Conductive bumps are formed, typically by printing of paste or any other means, to create a substantially conical bump. The interconnections are made in a two stage press. In a first pressing stage, a metal press plate is used to press the bumps to the prepreg so that they penetrate through the resin sheet layer, including the reinforcing layer. A secondary press stage is used to electrically connect the tips of the conductive bumps, previously pressed through the resin sheet, to a metal layer designed for making electrical contact with the bumps. To ensure plastic deformation of the bump tips, pressing plates on both sides are made of a material with little or no compression such as metal, heat resistant hard resin or ceramics. The plastically deformed-surface of the bump generates an inner, fresh active metal surface for bonding.
U.S. Pat. No. 6,705,003, by Motomura et al., discloses an additional step to the method of Yamamoto, of plasma cleaning the tip of the bump after the first press and before the second press. Even-height conical bumps are disclosed whose tips are deformed during the second press stage. Since the height of the bump has to be “substantially uniform”, this condition creates additional difficulties or additional processes in the creation of bumps through a bump plating process. In fact, most bumps are formed by creating a full conductive layer followed by etching away the unwanted metal to achieve the even height bumps. This is wasteful of the material used for the conductive layer.
Thus, conventional processes for forming vias on multilayer circuit boards typically require significant precision and expertise to achieve reliability and consistency. However, the inherent issues with microvias includes difficulties in processing, expensive processing machines, additional cost for use of specialty material such as laser drillable prepreg or resin-coated copper foil and chemical traps in the completed microvias. In addition, the size of microvias consumes much needed surface space on high-density, multilayer circuit boards.
Several attempts have been made to do away with microvias. These methods are not widely used due to (a) most of these alternative methods use a non-reinforced dielectric layer, (b) a metallization process is typically included to form the conductive layer over the dielectric layer, and/or (c) the pressing of substrates tends to be excessively complicated and require the conducting element to be of a fixed size and/or substantially uniform shape.